Studying Gorilla Behavior and Ecology

The Dian Fossey Gorilla Fund International is the leading organization studying mountain gorillas, and supports studies of Grauer’s gorillas as well. This research focuses on gorilla behavior and ecology, and is used to: inform conservation policies, enrich our knowledge of the great apes, and to learn how to protect them from extinction.

Studying mountain gorillas at Karisoke™

Since Dr. Dian Fossey established the Karisoke™ Research Center and began studying mountain gorillas in 1967, we have acquired an unparalleled amount of field expertise and data about mountain gorillas and their habitat, as well as the biodiversity of the region. Each year, Karisoke™ staff spend thousands of hours collecting basic information about the mountain gorillas, including their ranging patterns, changes in group composition (such as births, deaths, transfers), feeding and social behavior, health status, and major events (such as interactions among groups, group fissions, and dominance shifts). Our databases are some of the longest running and largest in the world and are used by us and our colleagues in Rwanda and internationally to answer critical questions concerning the basic biology of the subspecies and practical issues regarding their survival.

Much of what the scientific community knows about gorillas has come from the early research at Karisoke™ conducted in the 1970s and 1980s. Some of these discoveries include:

Mountain gorilla groups are structured around a dominant male, multiple females and their offspring. In the Virungas, about 40% of groups are multi-male but always led by a single, dominant male.

Both males and females often disperse from their birth group, with females immediately joining another group and males remaining solitary until they can attract females away from other males.

Group cohesion is maintained primarily through strong bonds between adult males and females, with relatively weak relationships within the genders. If the dominant male dies and there is no other male in the group to take his place, the group will usually disband.

Dominance relationships between females are relatively weak as a result of the low levels of feeding competition.

In groups with multiple males, the dominant male controls access to females, resulting in male-male competition being most apparent between rather than within groups.

Infanticide is a significant cause of infant mortality and is a reproductive strategy used by males to increase their reproductive success.

In recent years, the composition of the groups monitored by Karisoke™ has changed considerably from those first observed by Dian Fossey and early researchers. They have grown larger in size (from an average of 10-15 individuals to up to 65 individuals) and often contain significantly more males (up to 8 silverbacks as compared to 2 or 3 in the 1970s and 1980s). Consequently, we still have much to learn about gorilla social behavior and the factors influencing it. For example, how do larger groups affect the vegetation? Is there more competition for food in larger groups? Why do some males and females remain in their birth groups whereas others choose to leave? Does the dominant male still control breeding when there are a large number of other males in the group?

We are currently undertaking a number of studies using both our long-term databases and new data currently being collected to answer some of these questions. And we still have much to learn about other aspects of gorilla biology. Some of the recently completed and ongoing Karisoke™ studies of mountain gorillas include:

Factors Affecting Transfer Decisions in Male and Female Mountain Gorillas Karisoke™ scientists recently completed a series of papers, in conjunction with the Max Planck Institute for Evolutionary Anthropology, examining why female gorillas transfer between groups. The availability of the more than 40-year demographic database made this one of the most comprehensive studies on ape life- history decisions. The results differed from those of earlier studies, highlighting the importance of long-term data in illuminating this subject for long-lived species like gorillas.

Karisoke™ researchers have also published several papers on male gorillas, including male reproductive behavior in large, multi-male groups. A genetic analysis to see how the pattern of copulations and actual siring of offspring match up is currently underway, again in conjunction with Max Planck.

The Effect of Extreme Conservation Measures on the Virunga Mountain Gorilla Population’s Growth and Survival Our long-term demographic data have been merged with data on the remainder of the population of Virunga mountain gorillas to look at the growth patterns of the population as a whole over the last 40 years. The project has also investigated the role that extreme conservation measures, such as veterinary intervention and daily dawn-to-dusk protection, have played in saving this population. This project represents a collaboration among multiple research and conservation entities, including the Max Planck Institute, the International Gorilla Conservation Program, the Mountain Gorilla Veterinary Project, and the national park authorities in Rwanda, Congo and Uganda.

Feeding Ecology and Competition Among Mountain Gorillas The goals of this study are two-fold. Firstly, to evaluate the potential ecological basis for the higher density of gorillas in the Karisoke™ area, which now supports twice the number of gorillas as it did in the 1970’s and ‘80’s. This is done by comparing the density and abundance of plant foods consumed in the Karisoke area now to earlier studies (1970’s, late 1980’s and early 1990’s). Secondly, to examine the behavioral consequences of increased group size on group dynamics: specifically group spread, day journey length and female social relationships (dominance interactions and feeding competition). This is done by comparing females in groups of different sizes and assessing the links between social status, group size, and food intake.

GIS Analysis of Gorilla Ranging Patterns, Vegetation Classification, and Illegal Activities in the Virunga Conservation Area Karisoke™ Research Center staff collect GPS records on the location of gorillas as well as the location of snares, bamboo cutting, and other illegal activities in the park. These GPS points, along with historical information and maps, are entered into a computerized Geographic Information System (GIS), which helps us to understand how both gorillas and humans use the land. By analyzing poaching patterns in the Virungas since 1978, we can define optimal or suitable gorilla habitat and its implications for habitat management. These data are transferred each month to the national park authority (Rwanda Development Board, formerly ORTPN), and are used to inform park management activities.

Paternity Analysis To determine the paternity of individual gorillas, Karisoke™ trackers collect fecal samples from all individuals within the research groups for DNA analysis. This collaborative study with the Max Planck Institute will help us to answer many questions about mountain gorillas' reproductive behavior and mating patterns.

Social Dynamics and Reproductive Strategies of Male Mountain Gorillas In recent years, the number of male gorillas — blackbacks and silverbacks — within individual groups has risen. One of the questions we’re asking is: What is the impact of these larger multi-male groups on male behavioral patterns? And can we consider these changes to be adaptive? This study utilizes both traditional animal behavior data collection techniques and fecal and urine sampling to investigate male gorilla social dynamics and reproductive strategies.

Maternal Behavior and Infant Development A gorilla infant needs to suckle, as well as be groomed, transported, kept warm and generally protected. But how much time and energy is a gorilla mother willing to invest in her offspring? In collaboration with the University of Chester, UK, this doctoral study aims to throw light on the factors that shape the pattern of maternal investment and mother-offspring relationships in mountain gorillas. The study investigates areas such as gender, mother's social dominance, maternal experience, social environment and personality, as well as differences in the offspring's demands and needs. For this purpose, long-term behavioral records gathered over 30 years can be used, in addition to recent data.

Gorilla Personality Each gorilla has a different personality. This study looks at the relationships between individual gorilla personality and a variety of behaviors and life history patterns. For example – How does personality relate to different styles of group leadership or dominance? Do rank differences among adults correlate with personality differences? Does fertility or reproductive success correlate with personality? An understanding of mountain gorilla personality will allow us to better predict this species' capacity to adjust to population change (population size, number of groups, group density), and habitat change (change in food supply, loss of habitat, degree of human presence), and ultimately enable us to better conserve and manage this small, fragile population.

Census of the Mountain Gorilla Population During the spring of 2010, the first mountain gorilla census in seven years was carried out in the VIrunga Massif. The results, which were released eight months later, recorded 480 gorillas, which represents a 26.3 percent increase in the mountain gorilla population compared to the results of the previous census in 2003. The groups monitored and protected daily by the Karisoke™ Research Center in Rwanda's Volcanoes National Park included 126 individuals - a 17.4 percent increase from the number of gorillas we cared for in 2003.The increase in the Karisoke™ groups represents roughly 24 percent of the total increase in the population, yet these groups occupy only 10 percent of the Virunga range. Despite this increase, the mountain gorillas continue to remain a critically endangered species, and our mission to monitor and protect them is as vital as ever before.The 2010 census was a massive operation, which spanned two months (March and April) and the three countries of the Virunga Massif (Rwanda, Uganda and the DRC). It was a large collaboration between national park authorities, international conservation organizations and research institutes. The Fossey Fund played a major role in the census, providing 13 field staff (including two team leaders and three team leader assistants), as well as scientific expertise and guidance.

Tourism Impact Over 70 percent of the Virunga mountain gorilla groups have been habituated and many are visited by tourists on almost a daily basis. Although a number of specifically designed rules help to minimize the impact of regular tourist visits on the mountain gorillas, no studies had been conducted on the potential impact that tourists may have on their behavior and long-term survival. This study uses both behavioral and physiological data (urine and fecal samples) to assess the impact of tourism on the Virunga mountain gorilla population. It also looks at the impact our researchers might have on the three research groups they study. The first phase of the Tourism Impact Study found clear evidence of a negative impact of tourist visits on gorilla behavior. The gorillas were more aggressive and exhibited a number of stress-related behaviors during the one-hour tourist visits. During the visits, gorillas ate less, spent more time moving and traveled greater distances. The gorillas also spent less time doing their normal activities and a greater percentage of time watching the tourists, rather than interacting with one another. Although the long-term impact of these visits is still not known, the possibility that gorillas are experiencing higher stress levels due to the tourist visits suggests that this may result in reduced reproductive success and slower population growth. This study has provided the park management authorities the scientific information needed to guide sustainable long-term management of the gorillas in the face of increased economic pressure to include more gorilla groups in the tourism program and to increase the number of visitors and visits per day to each group.

Paternal BehaviourThe objective of this project is to assess the proximate mechanisms, form, and ultimate function of social relationships between adult male and immature mountain gorillas using a combination of behavioral, genetic, demographic, and hormonal data. The specific aims are: 1) to compare the forms of relationships between immatures, their mothers, and adult males in both single and multi-male mountain gorilla groups; 2) to investigate how hormones mediate males’ responses to infants, and assess the impact of relationship quality on the stress levels of immatures and their mothers; 3) to assess changes in male-immature relationships across time by combining new data from the proposed project with pilot data collected several years ago; and 4) to use all of these data to test alternative hypotheses about the function of male-immature relationships. Answers to these questions will speak to anthropological study of the origins of father-offspring relationships, from both an evolutionary (functional) and mechanistic perspective. Male-immature relationships in gorillas bear resemblances to their counterparts in human societies, and due to their social flexibility, they make an excellent comparative model with which to examine the origins of such relationships.

Gorilla StressTo date, there are no published physiological data on stress level variation in the Virunga mountain gorillas, factors contributing to stress and the implications of different stressors on the gorillas’ health and, in turn, their long-term survival. This project aims to provide the first understanding of stress physiology in the Virunga mountain gorillas and to undertake the first extensive examination of the relationship between stress and health in a wild ape. Our objectives are to: 1) to provide the first basic understanding of stress physiology in Virunga mountain gorillas considering age, sex, rank, seasonality and group characteristics; 2) investigate how naturally-occurring changes and events in the population (e.g. increasing number of intergroup interactions, growing group sizes and number of silverbacks residing in a group) as well as anthropogenic disturbances (e.g. poaching, human presence, veterinary intervention,climate change) contribute to stress; 3) understand how stress affects gorilla health by assessing the relationship between stress and immune function; 4) contribute to scientific capacity building in Rwanda; and 5) provide important information to the larger scientific and conservation communities to develop policies and make informed decisions on wild ape management and protection.

Female Hormonal studyThe goal is to investigate the hormonal correlates of socio-sexual behaviors in female mountain gorillas. Previous research has revealed much about reproductive behavior in mountain gorillas, but many questions remain that will be best addressed by simultaneously examining hormonal profiles and behavior. We’re focusing on two main aspects: reproductive strategies and fertility. Concerning reproductive strategies, we’re specifically focusing on understanding mating in relation to the time of conception, female mate choice, and male sexual coercion. For fertility, we are examining differences between nulliparous and parous females, adolescent sterility, sexual swellings, pregnancy loss, and age-related changes in fertility.

Studying Grauer’s gorillas in the Congo

In comparison to mountain gorillas, our knowledge of the Grauer’s subspecies is staggeringly little. Scientists do not even have a good estimate of the number of Grauer’s gorillas left, due to the size and remoteness of their habitat and the difficulties posed by ongoing armed conflict in many areas. They are estimated to number 5,000 or even fewer individuals. Thus, one of our primary goals is to document the distribution and density of the Grauer’s gorillas. We have worked in both a national park — Maiko National Park — and surrounding community reserves to conduct censuses aimed at providing data on where in their range Grauer’s gorillas are still located and how many there are remaining. Thus far, our work has found several populations of Grauer's gorillas that were previously unknown, in a corridor of forest that stretches between Maiko and Kahuzi Biega National Park.

The Dian Fossey Gorilla Fund has established a Grauer’s gorilla monitoring program in this area that employs 30 field staff working from a central field station and two mobile satellite camps located deep in Grauer's territory. They are focusing on key gorilla groups and collecting observations of their behavior and ecology, following the successful model provided by Karisoke. Developments in this program will be posted periodically on the Dian Fossey Gorilla Fund Web news pages and blog.